The overall objective of the proposed studies is to determine the physiological significance of alterations in the quantity and form of selenoproteins in milk. We will utilize human, animal and tissue culture studies to address the consequences of suboptimal selenium (Se) nutrition during lactation on mammary selenoprotein biosynthesis, milk lipid integrity and their ultimate utilization by the nursing neonate. We will focus principally on the relationship of Se & other dietary factors on the formation of glutathione peroxidase (GPx) and a recently identified 10 Kdal Se binding protein. Our detection of GPx in milk of several species suggests it has functional significance. We will isolate and characterize GPx in human milk and compare it to GPx in mammary tissue. To determine if GPx has functional role, we will utilize our animal model to study what effects dietary Se inadequacy has on GPx and selected selenoproteins within the mammary gland and their effects on lipid peroxidation within the gland and within milk. Oxidative stress, increased by polyunsaturated fatty acids, vitamin E deficiency and feeding autoxidized lipids, will be examined for its ability to modify GPx, other selenoproteins and lipid peroxides in mammary tissue and milk. The impact of such alterations on plasma & tissue lipids will be examined in the dam and her pup. We will also determine the characteristics of the 10 Kdal Se- binding protein, its content in mammary and other tissues. Polyclonal and Monoclonal antibodies will be formed to this protein and its reactivity with human mammary cells and plasma examined. Additionally, the cross-reactivity of this antibody with rat milk and tissue will be examined. Feeding studies will be conducted in rats to determine the influence of dietary Se intake on the quantity of this protein in rat tissues and milk.